1. Field of the Invention
The present invention relates to explosion vents for covering relief openings in enclosures subject to rapid pressure build-ups such as may occur during explosions or the like in bag houses, the duct work leading to the bag houses, or equipment upstream of the duct work. More particularly, the invention relates to an explosion vent that more consistently bursts or opens when the enclosure to which it is attached is subjected to a pressure build-up of a pre-determined magnitude without prematurely bursting at lower pressure levels or collapsing when the enclosure is subjected to conditions below atmospheric. The explosion vent is uniquely designed and configured to withstand continuous pressure cycling over an extended period of time wherein the individual pressure cycles are each insufficient to cause bursting of the vent.
2. Description of the Prior Art
Explosion vents are commonly used to cover relief openings in enclosures such as bag houses, tanks, etc. for preventing dangerous pressure build-ups within the enclosures. For example, bag houses are constantly at risk of explosions due to the high concentration of dust within the bag houses. Thus, bag houses are typically formed with a plurality of pressure relief openings, and explosion vents are placed over these openings. The explosion vents seal the openings when the bag houses operate at normal pressures and then burst or open when the bag houses are subjected to a pressure build-up of a pre-determined excess magnitude to uncover the openings and to vent the interior of bag houses. To prevent premature or late bursting, explosion vents must be designed to consistently burst at a particular pressure level.
Bag houses are also often subjected to vacuum conditions, particularly during the interval that their filters are being cleaned. Atmospheric pressure externally of the bag house causes an inward force on the explosion vents that tends to collapse the vents. Additionally, bag houses are often cycled between pressure and vacuum conditions, causing the explosion vents to flex back and forth. For example, it is common practice to direct pulses of air against the face of a bag filter which collects dust thereon, to dislodge the particles from the surface of the filter so that the particles thereby fall to a collection area below the filter bags. This cleaning of the surface of the filter bags results in pressure differentials being created within the bag house which result in pressure cycling of the protective vent. During such pressure cycling, the vent panel undergoes in and out movement. Thus, explosion vents must also be configured to withstand or resist vacuum pressures and cycling between pressure and vacuum conditions without collapsing inwardly into the enclosure.
Prior art explosion vents typically included a panel that was slit or formed with lines of weakness to define a rupture portion that ruptured or opened when subjected to a pressure build-up on one side thereof. A plurality of connectors or burst tabs were attached over the slit or lines of weakness to retain the panel in its closed position until subjected to a build-up of pressure of pre-determined magnitude.
Unfortunately, these types of prior art explosion vents frequently opened at pressure levels below or above their rated burst pressure levels because the panels did not uniformly distribute forces across all of the burst tabs, causing some of the burst tabs to break prematurely. Those skilled in the art will appreciate that when one or more of the burst tabs breaks prematurely, the remaining burst tabs are subject to breakage soon thereafter in accordance with a so-called "domino effect". To prevent such premature opening, vents have often been provided with additional burst tabs. However, this frequently caused the panels to open "late", or at pressure levels higher than their rated burst pressures.
It is not uncommon to employ bag house off time cycles which occur as frequently as every six seconds in order to permit cleaning air to be directed against the bag filter. In that instance, the vent panel will be exposed to over five hundred thousand cycles per year. Procedures for effecting cleaning of the filter elements of bag houses are described in detail in an article entitled "Optimize Pulse Jet Dust Collector Performance", published in Chemical Engineering Progress, August 1997, pp. 58-61, and in an article entitled "Five Ways to Upgrade your Pulse-Jet Bag House with the Latest Technology", appearing in Powder and Bulk Engineering, October, 1997, pp. 61-67. Rapid on and off cycling of bag house filter cleaning processes causes the explosion vents and their burst tabs to flex and bend back and forth, and can result in premature wear and breakage of the burst tabs. This means that the explosion vent for a bag house subjected to pressure cycles of an order of magnitude described must be replaced on a sufficiently frequent basis to avoid premature failure of the lines of weakness defining a rupture portion of the panel, depending upon the number of pressure cycles to which the vent panel is exposed during a defined period.
Another limitation of prior art explosion vents related to limited ability to withstand high vacuum pressures. Often, enclosures such as bag houses are subjected to vacuum pressures that are far in excess of the burst pressures at which the explosion vents are designed to rupture. These high vacuum pressures cause the burst tabs to break or cause the entire panel of the explosion vent to collapse inwardly. Therefore, when prior art explosion vents are used in such applications, they must be either reinforced, which increases their weight and cost, or used in conjunction with a separate vacuum protection panel.
The National Fire Protection Association (NFPA) has issued recommendations regarding weight limitations which in practice suggest that, if ferrous materials are used to fabricate explosion vents, said materials shall not exceed approximately 0.060 in. in thickness. This has imposed a significant limitation on the fabrication of explosion vents which are characterized by significant differential pressure parameters. The greater the pressure vacuum withstand value, then the thicker the vent material must be to meet the stringent differential pressure requirements.